Cranial nerves
Cranial nerves are nerves that emerge directly from the brain in contrast to spinal nerves which emerge from segments of the spinal cord. Although thirteen cranial nerves in humans fit this description, twelve are conventionally recognized. The nerves from the third onward arise from the brain stem. Except for the tenth and the eleventh nerve, they primarily serve the motor and sensory systems of the head and neck region. However, unlike peripheral nerves which are separated to achieve segmental innervation, cranial nerves are divided to serve one or a few specific functions in wider anatomical territories. Names of Nerves The 12 pairs of cranial nerves are traditionally abbreviated by the corresponding Roman numerals. They are numbered according to where their nuclei lie in the brain stem, e.g. Cranial Nerve III (the Oculomotor nerve) leaves the brainstem at a higher position than Cranial nerve XII, whose origin is located more caudally (lower) than the other cranial nerves. Nerves and nuclei As well as the visible nerves outside of the brain, most of the cranial nerves have associated nuclei within the brainstem. These nuclei are areas of grey matter, and damage to them can have a similar affect to the severing of an actual nerve. Axons to (and from) cranial nerves synapse first at the nuclei. Arrangement of the nuclei Just as grey matter in the ventral (closer to front of a human) spinal cord tends to be efferent (motor) fibers, and the dorsal horn tends to contain sensory neurons, nuclei in the brainstem are arranged in an analogous way. * Close to the midline are the somatic efferent nuclei, such as the oculomotor nucleus, which control skeletal muscle. Just lateral to this are the autonomic (or visceral) efferent nuclei (for instance, the Edinger-Westphal nucleus, which controls tears). * There is a separation, called the sulcus limitans, and lateral to this are the sensory nuclei. Near the sulcus limitans are the visceral afferent nuclei, namely the solitary tract nucleus. * More lateral, but also less posterior, are the general somatic afferent nuclei. This is the trigeminal nucleus. Back at the dorsal surface of the brainstem, and more lateral are the special somatic afferents, this handles sensation such as balance. * Another area, not on the dorsum of the brainstem, is where the branchial efferent nuclei reside. These formed from the branchial arches, in the embryo. This area is a bit below the autonomic motor nuclei, and includes the nucleus ambiguus, facial nerve nucleus, as well as the motor part of the trigeminal nerve nucleus. Cranial nerves in non-human vertebrates Human cranial nerves are evolutionarily homologous to those found in many other vertebrates. Cranial nerves XI and XII evolved in the common ancestor to amniotes (non-amphibian tetrapods) thus totalling twelve pairs. These characters are synapomorphies for their respective clades. In some primitive cartilagenous fishes, such as the dogfish (Squalos acanthos), there is a terminal nerve numbered zero (as it exits the brain before the first cranial nerve). Mnemonic devices As the list is important to keep in mind during the examination of the nervous system, there are many mnemonic devices in circulation to help remember the names and order of the cranial nerves. Because the mind recalls rhymes well, the best mnemonics often use rhyming schemes. The best known example is, "O'''n '''O'ld 'O'lympus' 'T'owering 'T'ops '''A 'F'inn 'A'nd 'G'erman 'V'iewed 'S'ome 'H'ops''," where 'A'nd represents auditory vestibular and 'S'ome represents spinal accessory. There are numerous mnemonics one example: "'O'hh, 'O'hh, 'O'h, 'T'o 'T'ouch 'A'nd 'F'eel 'V'irgin 'G'irls' 'V'aginas 'A'nd 'H'ymens" Another to help remember the types of information these nerves carry ('s'ensory, 'm'otor, or 'b'oth) is thus: *'S'''ome '''S'ay 'M'oney 'M'atters, 'B'ut 'M'y 'B'rother 'S'ays 'B'ig 'B'oobs 'M'atter 'M'ore.'' For additional memonics, see b:Transwiki:List of mnemonics for the cranial nerves Thirteen cranial nerves? Although twelve nerves are classically described in humans, there is a theory that finds favour with some authors, that humans really have thirteen cranial nerves (Andy Lelli , 1999). If the C1 spinal nerve were considered the thirteenth cranial nerve, C2 through C8 would be renamed as the first through seventh cervical nerves (though anatomists might change the term 'cervical' to 'nuchal' or some other term to avoid confusion between the two nomenclatures). This would make the numbering system consistent all the way through the spinal column - every nerve would exit below its corresponding vertebra, and the number of vertebrae would equal the number of spinal nerves. Alternatively, C1 could be considered the spinal root of the hypoglossal nerve (CN XII). This would, again, make the numbering system consistent. See also * Cranial nerve examination as part of [examination * Cranial nerve nucleus External links *Examination of the cranial nerves - University of Toronto Medicine * *CN Medical Notes on rahulgladwin.com *Information about the Cranial Nerves References Category:Peripheral nervous system *